Fix removal of all assignees

[koppor]

I changed the label from "good first issue" to "good second issue" at #12556 - and then the Assignee was removed -> https://github.com/JabRef/jabref/actions/runs/14022902800/job/39257172728

Should not happen - only removal if label "Assigned" or "FirstTimeCodeContribution" is removed.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [.] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [.] Tests created for changes (if applicable)
• [.] Manually tested changed features in running JabRef (always required)
• [.] Screenshots added in PR description (if change is visible to the user)
• [.] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [.] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

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